Whereas the milk of various species contains growth factors (GF's), such as epidermal growth factor (EGF) and insulin-like growth factors (IGF-I and -II), they are absent in infant feeding formulae. Biochemical and physiological studies in experimental animals indicates that milk is an important source of exogenous GF-s for the suckling. Since it is known that transforming growth factor alpha (TGFalpha) exhibits similar effects as EGF, comparative studies using TGFalpha will be included. We will study the potential to synthesize EGF, TGFalpha, IGF-I and -II of individual organs of suckling rats at various postnatal developmental stages and compare the organ level of their peptide GFs, as well as EGF- receptor levels and corresponding messenger RNA (mRNA). We will explore whether the levels of these peptides and corresponding mRNA depend on milk-borne GF's, using rats fed rat milk substitute (RMS) containing no GF or as supplemented with GF. Our previous experiments have demonstrated presence of various forms of EGF in milk, the stomach (so-called gastric form), and small intestine (so-called intestinal form) of suckling rats, therefore, we will compare their biological activity.